The universal serial bus (USB) has a variety of operating modes that allow a number of computer peripherals to be connected to a generic port. One of the modes of a USB device is a low speed mode. While operating in the low speed mode, the outputs of a USB device are generally very slow transitioning signals. One design criteria involved with USB devices is that there may be a large variation in the load placed on the device. As a result, the USB device must be designed to provide the proper speed at the output across wide load variations.
The USB specification calls for controlled differential rise and fall times over a 7:1 range of load capacitances (i.e., from 50 pf to 350 pf) for operation in the low speed mode. The USB specification also requires that the differential outputs cross between 1.3 and 2.0 volts over the entire load range. A particular design problem in implementing an output driver to meet the crossover specification occurs since the negative output signal generally contains a 1.5 K.OMEGA. pull-up resistor at the output. The pull-up resistor causes the negative output to rise more quickly than the positive output causing an imbalance in the output crossovers. The crossovers are also effected by process and temperature variations. As a result, the circuit must not only change the signal delay, but must also track the process and temperature variations.
Referring to FIG. 1, a conventional USB driver circuit 10 is shown which implements operational amplifiers 12 and 14 to control the transitioning of the output signal. The circuit 10 generally requires the use of the operational amplifier 12 and the operational amplifier 14 to control the signal presented at an output pad 16. The circuit 10 also requires a variety of transistors, a bias circuit 18, a predriver circuit 20 and a clamp circuit 22. More information regarding the circuit of FIG. 1 may be found in the paper "Design Guide For A Low Speed Buffer For The Universal Serial Bus," published in December 1996, the relevant sections which are hereby incorporated by reference.
Each universal serial bus device has two outputs, a plus output and a minus output. The circuit 10 would have to be implemented, at a minimum, at each of the plus and minus outputs of each device. In an application that provides a number of universal serial bus outputs on a single device, the number of instances that the circuit 10 would have to be duplicated increases accordingly. As a result, it is desirable for the number of components in the circuit 10 be kept to a minimum in order to reduce the overall area required to implement the plurality of required buffers at the various outputs.
The USB specification also requires that the differential outputs be monotonistic (i.e., smoothly transitioning). When switching from a HIGH (e.g., "1") to a LOW (e.g., "0") in low speed mode, the pull-up resistor on the D- output signal generally causes the D- output to pull toward the power supply voltage until the NMOS pull-down transistor reaches its threshold voltage. This may cause a variation in voltage, or a "bump," to occur on the output. It is generally desirable to reduce the bump at the output.